Injectors of this kind comprise a conveying jet nozzle, a jet catching duct opposite and axially spaced from the nozzle, and a powder-aspirating aperture affixed to the powder jet nozzle or between it and the jet catching duct. Furthermore one or more apertures for additional air may be present at the conveying-jet nozzle or between it and the jet catching duct or downstream of the latter. Injectors of this kind are known in various embodiments from the German patent documents 1,266,685; 1,922,889 and C2 42 01 665. The jet catching duct is frequently also called "catching nozzle".
The known injectors incur the drawback that the airflow and the powder particles wear down the jet catching duct. As a result, not only must the jet catching duct be exchanged frequently, but also, depending on the rate of wear, the volumetric powder flow (quantity of powder conveyed per unit time) will change, entailing non-uniform coating thicknesses of a workpiece being coated. The known jet catching ducts are made of metal or plastic. Metal ducts are is disadvantageous in that the powder particles tend to adhere and incipiently sinter at the duct wall. As a result the cross-section of the guiding duct varies and so does the volumetric powder flow. Plastics offer better slippage than metal, and typically those plastics are selected for which the powder particles show minimal adhesive friction. But in this design as well powder particle accretion and sintering onto the plastic duct walls cannot always be reliably avoided. Moreover plastics are "softer" than metal and accordingly suffer more from wear.
An objective of the invention is to achieve a lesser rate of wear of the jet catching duct and reducing, or even completely avoiding accretions and sintering onto the duct of powder particles.